It is known that a conventional variable valve operating device described, for instance, in Patent Document 1 mechanically changes the operating angle and lift amount of a valve in accordance with the operating state of an internal combustion engine.
In the variable valve operating device described in Patent Document 1, two rotary cams are installed over a camshaft. Two intake valves are provided for a single cylinder. A first intake valve is opened and closed by a first rotary cam. A second intake valve is opened and closed by a second rotary cam. A variable valve transmission mechanism, which includes a four-joint link mechanism, is positioned between the first rotary cam and the first intake valve and between the second rotary cam and the second intake valve.
The above variable valve operating device can continuously vary the lift amount of the two intake valves. Therefore, an internal combustion engine having the above variable valve operating device can conduct a so-called non-throttle operation in which the intake air amount is controlled in accordance with changes in the intake valve lift amount without using a throttle valve.
Further, the above variable valve operating device includes a switching mechanism that uses a coupling pin to couple the four-joint link mechanism for the first intake valve to the four-joint link mechanism for the second intake valve or uncouple them from each other. The switching mechanism can select either a dual valve variable control mode or a single valve variable control mode. In the dual valve variable control mode, the lift amounts of the first and second intake valves are simultaneously varied. In the single valve variable control mode, on the other hand, the lift amount of only the first intake valve is varied while a large lift amount is constantly provided for the second intake valve.
The above variable valve operating device can exercise swirl control to create a swirl flow in a combustion chamber in the single valve variable control mode in which the lift amount of the first intake valve differs from that of the second intake valve so that the first and second intake valves differ in the air flow rate. A swirl flow in the combustion chamber brings about combustion improvement, for instance, in a low-load condition.    Patent Document 1: Japanese Patent Laid-open No. 2004-100555